Charged-particle-γ and charged-particle-neutron-γ coincidences have been used to identify in-beam γ-ray transitions in ${}_{}{}^{69}{}_{}{}^{}\mathrm{Se}$ produced in the reaction ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$${(}^{32}$S,2pn${)}^{69}$Se. Charged-particle-γ-γ coincidences, neutron-γ-γ coincidences, and neutron-gated γ-ray angular distributions were used to deduce the level scheme. A long-lived isomer attributed to the $g_{9/2}$ state was found; its half-life was measured with charged-particle-γ timing as 1023±110 ns. The presence of strong ΔJ=1 and ΔJ=2 transitions in the band built on the ν$g_{9/2}$ neutron single-particle state and the measured sign of the E2/M1 mixing ratio for the ${(11/2\mathrm{}}^{+}$→${(9/2\mathrm{}}^{+}$ transition are consistent with a theoretically predicted oblate shape for this nucleus.

• Received 30 May 1989

DOI:https://doi.org/10.1103/PhysRevC.40.2010